This invention relates to processes for the removal of acid and basic gases from contaminated industrial process waters; but more especially to the removal of ammonia and acid gases, such as carbon dioxide and hydrogen sulfide from such industrial process waters.
Many important industrial processes, e.g., petroleum refining, coking effluents from steel processing, and emerging technologies, e.g., coal gasification and oil shale retorting, produce effluents, or recycle water streams wherein ammonia and one or more weak acid gases, e.g., carbon dioxide, hydrogen sulfide, hydrogen cyanide, etc., are dissolved together. Environmental protection regulations require the removal of these dissolved gases before the release of such waters. In other instances, these dissolved gases must be removed before the waters are fit for recycling. In addition, where the volume of these waste or recycle waters is high and the percentage of dissolved gases is substantial, there may be economically worthwhile amounts of these contaminants available for recovery. Thus ammonia is a valuable industrial commodity, as is sulfur, etc. Therefore even without environmental considerations, it may be advantageous to recover these contaminating gases.
In the past, the most common method of recovering the contaminants has been the separation of gases from the waste, or recycle waters, by steam stripping. Unfortunately, however, both the basic ammonia gas and the acidic gases ionize in solution, and the pH stabilizes out to a value typically about 8 to 9. At such pH's both the ammonia and acid gases are substantially ionized. When ionized, the volatility of the gases are substantially reduced. Low volatility greatly increases the steam requirement to effectuate stripping. Boil-up ratios of 0.08 to 0.15 lbs. of steam per pound of water are commonly encountered. In cases where it is desired to isolate the ammonia as a pure product, even more elaborate stripping processes are required. These more elaborate processes further increase the capital costs and add to the steam requirements. Of course, the generation of steam is energy intensive, and increased fossil fuels costs in recent years have sparked interest in techniques for reducing the steam requirements for such processes, or devising other techniques for removing the contaminating gases.
One technique that has been studied for the more efficient removal of basic and acid gases from process waters involves the use of liquid-membrane processes in combination with traditional stripping techniques. For instance, Cahn et al., in Environmental Science and Technology, 12, 1051 Et.seq. (1978), and in U.S. Pat. No. 4,029,744 issued June 14, 1977, describe the removal of weak bases and weak acids by contacting the contaminated solution with a specially devised water-in-oil emulsion. In this technique one of the contaminating species permeates the emulsion's external oil phase and then reacts with a reactant present in the interior phase to produce a neutralized product. This neutralized product effectively removes one of the contaminants and provides a continuing driving force for the further permeation of the emulsion by the one contaminant. Thus one contaminant is effectively removed. At the same time, the other contaminants are continually stripped by conventional steam stripping or air blowing of the feed waters.
Although the liquid-membrane technique shows promise, other energy efficient processes would also be of considerable interest.